Field of the Invention
The present invention relates to a printed circuit board on which a piezoelectric element is mounted, a power supply apparatus using the printed circuit board, and an image forming apparatus including the power supply apparatus.
Description of the Related Art
Flow soldering is known as a method of soldering an electronic component to an electronic device. The flow soldering method performs soldering by applying flux to a printed circuit board on which an electronic component is mounted, and dipping the board into a flow solder bath containing molten solder. More specifically, soldering is performed by forming a jet flow of molten solder (solder jet flow) in the flow solder bath, and bringing the board into contact with the top of the solder jet flow.
During preheating and passage through the flow solder bath in the flow soldering process, a piezoelectric transformer is heated to as high as several hundred degrees, and a high voltage is generated at the terminal owing to the pyroelectric effect. More specifically, a spark discharge occurs at a gap formed between a primary side terminal serving as the driving side of the piezoelectric transformer and a soldering land. A discharge voltage at this time reaches almost several hundred to several thousand V. To the contrary, the electrostatic breakdown voltage of the terminal of a semiconductor component such as an LSI or transistor is about several ten to several hundred V at most. When a discharge occurs due to the pyroelectric effect, a semiconductor component coupled to the extension of the soldering land at the primary side terminal of the piezoelectric transformer may break due to electrostatic dielectric breakdown.
Japanese Patent Laid-Open No. 2009-130311 proposes a method of arranging short-circuit terminals at two primary side terminals of a piezoelectric transformer and temporarily short-circuiting them by a conductive jig. This can suppress generation of a high voltage owing to the pyroelectric effect in the flow soldering process.
Japanese Patent Laid-Open No. 2000-307166 proposes a method of parallelly soldering a resistive element between the primary side terminals of a piezoelectric transformer, thereby suppressing generation of a high voltage owing to the pyroelectric effect. In this method, a pyroelectric current generated by the pyroelectric effect is discharged via the resistive element, reducing a voltage rise between the primary side terminals of the piezoelectric transformer. This suppresses a voltage applied to a semiconductor component coupled on the primary side of the piezoelectric transformer.
In the invention of Japanese Patent Laid-Open No. 2009-130311, the conductive jig needs to be reliably mounted between the primary side terminals to neither drop during flow soldering mounting nor cause a contact failure. Repetitive short-circuiting in a high-temperature environment requires a conductive jig which is excellent in heat resistance and durability and is easy to attach and detach. Depending on the conditions of the temperature and transfer speed in the flow soldering process, a pyroelectric voltage generated by the pyroelectric effect exceeds the electrostatic energy breakdown voltage of a semiconductor component. To prevent this, conditions to suppress the pyroelectric voltage need to be defined.
A resistive element described in Japanese Patent Laid-Open No. 2000-307166 is soldered to a circuit on a board only after passing through the flow solder bath in the soldering process. Hence, this method cannot satisfactorily suppress generation of a pyroelectric voltage upon an abrupt temperature rise of the piezoelectric element during preheating or passage through the flow solder bath in the soldering process. The electrodes of the primary side terminals of the piezoelectric transformer need to be reliably coupled before preheating and arrival at the flow solder bath.